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2017 Spring Science Colloquia

Constraining Ultra-Light Axions with CMB data: Sharing Light on the Lightest Particle Dark Matter Candidates

Presented by: Renee Hlozek  (Dunlap Institute, University of Toronto)
Category: Science Colloquia   Duration: 1 hour   Broadcast date: March 01, 2017
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CMB cosmology is currently undergoing a data-rich epoch, with measurements on small scales from experiments like the Atacama Cosmology Telescope (ACT) and its polarisation instrument, ACTPol, adding to measurements on larger scales by Planck, WMAP and most recently BICEP. I will contextualise the measurements and present constraints on cosmological models of interest to small-scale experiments; while noting that foregrounds that complicate our measurements of the primordial microwave sky. I'll present recent ACTPol results and highlight the next generation of the experiments in the Simons Observatory and the Fourth Stage CMB experiment (CMB S4): ensuring CMB cosmology has a rich future to look forward to on the ground! I'll also concentrate on how we can use the CMB to constrain the lightest potential dark matter particles. I'll pay particular attention to constraints on models of ultra-light axions (ULAs). ULAs are a type of dark matter well motivated by particle physics, that impact our cosmological measurements by suppressing structure and affecting the CMB. I'll highlight previous work which constrained ULAs with mass in the range 10-^{32} eV?10^{-26} eV, current limits are that they can compose a fraction <0.01 of the cosmological critical density of the universe. CMB-S4 should be ?10 times more sensitive to the ULA energy-density than Planck data alone, across a wide range of ULA masses 10-32<ma<10-23 eV, and will probe axion decay constants of fa?1016 GeV, at the grand unified scale. It should also improve the CMB lower bound on the ULA mass from ?10-25 eV to 10-23 eV, nearing the mass range probed by dwarf galaxy abundances and dark-matter halo density profiles. These improvements will allow for a multi-? detection of percent-level departures from CDM over a wide range of masses.